Lithographic apparatus, device manufacturing method

ABSTRACT

To compensate for birefringence of a mask in a lithographic projection apparatus, the birefringence of a mask is measured and stored as birefringence data in a data storage device. A birefringent compensation element is disposed in the optical path of the lithographic projection apparatus. Appropriate adjustments of the compensation element are determined as those optimally reducing impact of the mask birefringence on the state of polarization at substrate level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus, and a devicemanufacturing method.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,comprising part of, one, or several dies) on a substrate (e.g., asilicon wafer). Transfer of the pattern is typically via imaging onto alayer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions that are successively patterned. Knownlithographic apparatus include so-called steppers, in which each targetportion is irradiated by exposing an entire pattern onto the targetportion at one time, and so-called scanners, in which each targetportion is irradiated by scanning the pattern through a radiation beamin a given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti-parallel to this direction. Itis also possible to transfer the pattern from the patterning device tothe substrate by imprinting the pattern onto the substrate.

The imaging of the pattern involves illuminating the patterning devicewith electro magnetic radiation. With high Numerical Aperture (NA)projection systems and with high NA immersion projection systems fortransfer of the pattern, it is desirable to provide polarized or atleast partially polarized illumination radiation. This enables imageformation at wafer level by radiation with a state of polarization whichis suitable for optimal contrast of the image. For example, where imageforming radiation has a p-polarized, an s-polarized and an unpolarizedcomponent, it is in particular the s-polarized radiation component whichcontributes most to contrast of the image. Consequently an illuminationsystem for use with a lithographic apparatus may therefore be arrangedto specifically provide s-polarized illumination radiation. However,optical elements downstream of the illumination system may destroy oraffect the state polarization of radiation traversing these elements dueto, for example, the presence of residual or inherent opticalbirefringence in the material of the elements or due to effects ofoptical (single or multi-layer) coatings on the element surfaces. Duringassembly of an optical system of a lithographic apparatus parametersaffecting a state of polarization can be monitored andtolerance-controlled. However, optical elements not being a fixed partof the optical system, such as for example a patterning device embodiedas a reticle, may cause a depolarization or a change of polarizationaffecting image contrast beyond tolerance.

SUMMARY OF THE INVENTION

It is desirable to have an improved control over the state ofpolarization of radiation in a lithographic apparatus at substratelevel.

According to an aspect of the invention, there is provided alithographic apparatus arranged to project a pattern from a patterningdevice onto a substrate using an electro-magnetic radiation beam,comprising:

a data storage device arranged to receive and store data characterizinga birefringence property of the patterning device,

a manipulator for adjusting one or more birefringent elements in usetraversed by said radiation beam,

a controller responsive to said data and arranged to control saidadjusting to compensate impact of the patterning device on apolarization state of said radiation beam at substrate level.

According to another aspect of the invention, there is provided a devicemanufacturing method comprising projecting a pattern from a patterningdevice onto a substrate using an electro-magnetic radiation beam, themethod comprising:

storing data characterizing a birefringence property of the patterningdevice,

adjusting one or more birefringent elements in use traversed by saidradiation beam,

controlling said adjusting to compensate impact of the patterning deviceon a polarization state of said radiation beam at substrate level.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2A depicts a mask with local fast and slow axes characteristic forbirefringence;

FIG. 2B illustrates impact of a birefringent mask on a state ofpolarization of radiation traversing the mask;

FIG. 2C illustrates compensation of mask birefringence using a separatebirefringent element;

FIG. 3 depicts a slit shaped illuminated area on a mask when used with ascanner;

FIG. 4 illustrates an array of birefringent elements for compensation ofbirefringence of a mask according to an embodiment of the invention, anda plot of phase retardation along an axis of a birefringent elementaccording to an embodiment of the invention;

FIG. 5 illustrates a distribution of fast axis directions of abirefringent mask;

FIG. 6 illustrates a birefringent element for local compensation of fastaxis variation of direction along the scanning direction of a maskaccording to an embodiment of the invention;

FIG. 7 illustrates a mask birefringence measurement device.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g., UV radiation such as for example generated by anexcimer laser operating at a wavelength of 193 nm or 157 nm radiation);

a support structure (e.g., a mask table) MT constructed to support apatterning device (e.g., a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters;

a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., comprising one ormore dies) of the substrate W;

a data storage device DS arranged to receive and store datacharacterizing a birefringence property of the patterning device;

a controller CN responsive to data stored in the data storage device DS;and

a manipulator MN for disposing a birefringent element BE in theradiation beam B and for adjusting the birefringent element BE.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e., bears the weight of, thepatterning device. It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, and catadioptric optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, or forother factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g., an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g., so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g., after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion. For reference in the text below, ascanning direction is the Y-direction as defined by the coordinate axesshown in FIG. 1. In the present context, an X,Y,Z-coordinate system isfixed with respect to the apparatus. Similarly, x- and y-axes areassociated with a reticle and a mask pattern, whereby for a reticle foruse with a scanner the direction of the y-axis corresponds with theY-direction in which that reticle is scanned.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Reticles for use with projection lithography typically consist of aquartz substrate, at one surface provided with the pattern to be imaged.Crystalline quarts is known to be birefringent, therefore reticle blanksare usually made of fused quartz. However, even if pulsed quartz is usedfor the reticle blank, the presence of some residual birefringence ispractically unavoidable. As shown schematically in FIG. 2A, thebirefringence of a portion of a reticle, referred to hereinafter as“reticle birefringence” and denoted by RB, can be modelled in a point(x,y) on the reticle MA as the birefringence of a retarder plate with afast axis FA-MA and a slow axis SA-MA and with a phase retardation whichis usually indicated as a fraction of the wavelength. In general, thereticle birefringence RB is a function of the reticle coordinates x andy. Both the orientation of the fast and slow axes (FA-MA, SA-MA) and thephase retardation may have an x,y distribution of values. This x,ydependence is expressed as:RB=RB[A,N],A=A(x,y), andN=N(x,y).

Here, A is the angle between the y-axis and the fast axis, and N is thephase retardation expressed in fractions of the wavelength of theradiation beam B.

The effect of the angle A on the state of polarization of radiationtraversing the reticle in a point (x,y) on the reticle MA isschematically illustrated in FIG. 2B. The input polarization of anincoming ray 20 of light is a linear polarization state. In FIG. 2 theincoming radiation is linearly polarized parallel to the y-axis, asillustrated by the arrow 21. The fast axis FA-MA at a point (x,y) ofintersection with the reticle of the incoming ray subtends an angle Awith respect to the y-direction, and the impact of the presence of thereticle on this ray is that it emerges from the reticle with a resultantelliptical polarization, as illustrated by the ellipse 22. Theparameters describing the exact shape and orientation of the ellipse 22depend on the angle A(x,y) and the phase retardation N(x,y) at the point(x,y). The effect of diffraction of radiation at features (not shown inFIG. 2B) of a mask pattern present near the point (x,y) is not changingthe effect of birefringence on the state of polarization: resultingdiffracted rays all have said elliptical polarization. A pattern withfeatures extending along the y direction leads to diffracted obliquerays in the x-z plane. In the absence of birefringence these obliquerays would be p-polarized. The impact of the reticle is that imageformation is now accomplished with elliptically (and hence not fullyp-polarized) radiation resulting in a decrease of image contrast.

Data describing A(x,y) and N(x,y) can for example be obtained bymeasurement of the reticle birefringence, and can be provided to thedata storage device DS. In the present embodiment these data are used tofirst calculate the average values of A and N, to be denoted by Aav andNav, as obtained by averaging these values over an area of the reticleMA which comprises the mask pattern.

An impact of a reticle with an average birefringence characterised byAav and Nav on the polarzation of radiation traversing that reticle canbe compensated by disposing in the path of the radiation beam B aretarder plate BE with a retardation chosen equal to Nav, and with itsrotational orientation with respect to the z-axis chosen such that theslow axis SA-BE of element BE is parallel to the average direction Aavof the fast axis FA-MA characteristic for the reticle birefringence.This arrangement of element BE and the reticle MA is illustrated in FIG.2C, and results in the combination of the reticle MA and the retarderplate BE having an average zero (or at least substantially reduced)phase retardation. Hence, the impact of the reticle on the polarizationstate at wafer level is reduced, and image contrast improved.

Selected reticle birefingence data stored in the data storage device DSrepresentative for A(x,y) and N(x,y) are retrieved by the controller CN.The controller CN is arranged to calculate the average values Aav andNav, to identify from a plurality of available, preselected crystallinequartz retarders (with a corresponding preselected plurality of phaseretardations) a retarder BE which most closely matches the phaseretardation Nav and to instruct the manipulator MN to obtain theselected retarder BE and dispose this retarder BE in the path traversedby the radiation beam B. Each retarder BE is calibrated in relation tothe direction of its fast axis FA-BE and its slow axis SA-BE. Themanipulator MN comprises a motorized rotatable mount for holding aretarder plate BE in a preselected rotational orientation (of its fastand slow axes) and is arranged to rotate the selected retarder BE suchthat its slow axis SA-BE aligns with the the direction Aav of thereticle in response to a signal repesentative for Aav. The amount ofrotation needed for this alignment is determined by the controller CNand can be applied to the retarder BE by the manipulator MN before,during or after disposing the retarder BE in the optical path.

Birefringence data for a plurality of reticles can be stored in the datastorage device DS, so that during usage of a particular reticle thecorresponding appropriate compensation of birefringence can be providedin a manner as described above.

The location along the optical path where a compensating retarder BE isplaced in the optical path is not critical. It is primarily determinedby aspects such as the space available in the apparatus for themanipulator MN, and the lateral extent of the required clear aperture ofthe retarder element BE. The latter parameter is an optical systemdesign parameter, and is different for different locations along theoptical axis. The required clear aperture at a location near the reticlefor a stepper respectively a step-and-scan apparatus is such that themask pattern respectively the slit shaped illuminated area is entirelyenclosed within said clear aperture.

The smaller the clear aperture at a location along the optical path, thesmaller the diameter of the retarder element can be. Since the cost of acrystalline quartz retarder element BE increases with its diameter,there is a preference to locate the retarder BE at a location where therequired clear aperture is relatively small. With apparatus employing arod-type optical integrator, the aperture of the exit face of theintegrator rod is a relatively small clear aperture. The embodiment ofthe invention illustrated in FIG. 1 schematically shows the placement ofthe retarder BE at this preferred location.

However, it will be appreciated that the retarder position can also bechosen, for example, in a plane conjugate to the reticle or near thereticle itself (either upstream or downstream of the reticle), or in apupil plane of either the illumination system IL or the projection-lensPS.

Instead of having a plurality of preselected retarders available, it ispossible to use just one retarder BE if the birefingent properties Aavand Nav of a relevant group of reticles are distributed within asufficiently small bandwidth.

According to an aspect of the invention, the polarizing element BE maybe embodied as a variable phase retardation plate. Examples of variablephase retardation plates are Babinet and Soleil compensators arranged toadjust the phase retardation to a preselected value of phaseretardation. The manipulator MN comprises, besides said rotatable mount,a drive for setting the variable retarder plate to a desired phaseretardation in accordance with a signal provided by the controller CNrepresentative for the average phase retardation Nav which is to becompensated. With this embodiment, the need for having available aplurality of retarder plates with a corresponding plurality of phaseretardations is alleviated. Electro-optic and piezo optic materials canalso be used as retardation plate substrate, because their birefringencecan be changed by respectively varying an electric field or pressurewithin the substrate.

In an embodiment of the invention the lithographic apparatus is ascanner and the manipulator MN is arranged to adjust a plurality ofbirefringent elements such as to compensate a field dependentpolarization impact of the reticle on the polarization state of theradiation beam at substrate level. In a scanner the illumination systemIL is shaping the radiation beam B such that a slit shaped field 30 onthe reticle MA is illuminated, see FIG. 3. The slit extends along the Xdirection, perpendicular to the Y direction corresponding to thescanning direction. Its width is denoted by SLW, as shown in FIG. 3, andis fixed with respect to the X,Y,Z coordinate system.

Any x-dependence of the reticle birefringence RB may cause a fielddependent polarization impact of the reticle on the polarization stateof the radiation beam at substrate level (in the X-direction). Althoughsimilarly an y-dependence of RB causes a field dependent polarizationimpact in the Y-direction at substrate level, in many instancesbirefringence variations of the reticle over distances of the order ofthe width SLW can be ignored or are within tolerance, whereas variationsover distances of the order of the length of the slit may be beyondtolerance. As is illustrated in FIG. 4, to compensate for such out oftolerance variations a polarizing element BE is in the presentembodiment arranged as an assembly 40 comprising a plurality ofbirefringent fingers, i.e., a plurality of elongated birefringentelements BEi, (index i running from 1 to the total number of fingers).The birefringent elements BEi have a common direction of elongation, forexample along the scanning y-direction, and are disposed parallel tosaid common direction of elongation. Further, they are mutuallydisplaced with respect to each other in a direction perpendicular tosaid common direction of elongation. Each birefringent element BEi isarranged movable along its direction of elongation, as illustrated byarrow 41 in FIG. 4, and each birefringent element BEi has a non-uniformdistribution of one or more birefringence properties along the axis ofelongation.

For example, a birefringent element BEi may have a phase retardationNi(y′) which is linearly varying along the length of a finger. Here y′is a coordinate associated with the finger, along the axis ofelongation, see the N-y′ plot in FIG. 4. A finger made out of abirefringent crystalline quartz substrate and provided with a wedgeshape (such as to obtain a linearly varying thickness along its longdimension) provides such a non-uniform (linearly varying) distributionof phase retardation along its axis of elongation.

The assembly 40 of fingers may be placed near the reticle, andalternatively or in or near a plane optically conjugate to the reticle,so that in a static situation only a part of each finger is irradiated(as illustrated in FIG. 4) and an adjustment 41 can effectively providea local change of a birefringence property.

For each element BEi data representative for the particular non-uniformdistribution of said one or more birefringence properties along the axisof elongation are stored in the storage device DS. For a finger BEithese data may, for example, represent values of Ai(y′) and/or Ni(y′).The reticle birefringence data RB can be provided as values RB(A(xi,yi), N(xi, yi)) for a grid of points (xi, yi) on the reticle MA. Thecoordinates xi can be chosen to coincide with the x-positions of thefingers BEi. During a scan, the slit area 30 will subsequently traversethe array of coordinates yi. Therefore, coordinates yi representdifferent scan positions of the reticle with respect to the illuminatedslit area 30. For each scan position yi the controller CN calculates aplurality of desired positions of the plurality of fingers BEi withrespect to the slit area 30 whereby the properties Ai(y′) and Ni(y′) ofthe part of the fingers BEi which is irradiated are optimallycompensating corresponding local reticle birefringence's RB( A(xi, yi),N(xi, yi)). The manipulator MN adjusts the settings schematicallyindicated by arrow 41 in FIG. 4 in accordance with the calculatedplurality of desired positions for the elements BEi.

According to an aspect of the present embodiment, the slow axes of thefingers may be embodied parallel to each other, and the phaseretardation for each finger may be a linear function of y′, as describedabove. The assembly 40 of fingers is arranged in a rotatable mount whichis part of the manipulator MN, so that in principle alignment of saidslow axes with an average direction Aav of a reticle fast axis FA-MA ispossible(within a limited range of directions Aav). Further, byoptimally positioning the elements BEi any out of tolerance phaseretardation N(x,y) of a reticle can be compensated for the whole field,during scan.

A typical reticle birefringence distribution is schematicallyillustrated in FIG. 5. Contrast loss in an image is primarily caused byan x,y-variation in the orientation of the local fast axis A(x,y),whereas the phase retardation N(x,y) is sufficiently narrowly peaked atan average value Nav to consider it as constant, independent of x,y.Hence, in order to compensate this type of reticle birefringence,modelled as RB(A(x,y), Nav), each finger BEi preferably has a phaseretardation Nav, and the direction of the slow axis (Ai(y′)+π/2)preferably varies across the finger in accordance with the subsequentvalues for A(xi, yi) which are affecting the polarization state at waferlevel when during scan corresponding subsequent reticle points (xi,yi)are illuminated.

As schematically illustrated in FIG. 6, one can identify a number ofdiscrete fast axis directions Ai on a line with xi=constant, which areto be accounted for in the finger BEi. The finger BEi which is tooperated and adjusted along the line xi=constant in FIG. 6, may beembodied as an array of birefringent crystalline quartz segments 62,each having a phase retardation Nav, whereby the slow axes ofconsecutive segments 62 are aligned with corresponding consecutivedirections of local fast axes at the reticle along the line xi=constant.In FIG. 6 this is illustrated by the line segments 61 (representing aslow axis direction) being parallel to the line segments 50(representing local fast axis directions at the reticle).

The birefringent elements BE and also said fingers BEi in any of theabove embodiments can be made of birefingent materials transmissive forradiation of the wavelengths mentioned above, such as for examplecrystalline quartz, CaF₂, and MgF₂.

In any of the embodiments described above, the lithographic apparatusmay further comprise a birefringence measurement device for measuringthe birefringence distribution RB(x,y) of a patterning device such as areticle. Measurement results can be obtained prior to exposure of thereticle and stored in the device DS. The birefringence measurementdevice may be part of a reticle handling system or of the reticle stage.FIG. 6 schematically illustrates a birefringence measurement device BMD.The device is of a reflective type; a HeNe laser, not shown in FIG. 7,generates a probe beam 74 which is reflected at chrome features 70forming the mask pattern on the mask MA and which, upon reflection atthe mask surface is captured by a radiation detector (not shown in FIG.7). A common polarization measurement set-up comprises a linearpolarizer, a quarter wave plate, and an analyzer, respectively elements71, 72 and 73 in FIG. 7. With the HeNe laser generated probe beam 74 themask substrate is sampled with a polarization measurement set-up wherebythe polarizer 71 is in 45 degree with the quarter wave plate. The angleof rotation around an axis coincident with the probe beam of theanalyzer gives the information on the reticle birefringence. Otherbirefringence measurement methods (in transmission or reflection) may beused as well.

According to an aspect of the invention, the birefringence measurementdevice BMD system comprises an alert system which generates a flag toalert a user of the apparatus in case a probed reticle is beyondtolerance in respect of reticle birefringence. Alternatively, alithographic apparatus can be equipped with said birefringencemeasurement device BMD comprising an alert system, without furthercomprising means to provide a compensation of reticle birefringence. Theuse of reticles beyond tolerance in respect of reticle birefringence canthan be avoided.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion,” respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm).

The term “lens,” where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, and reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practised otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g., semiconductor memory, magnetic or optical disk) havingsuch a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus arranged to project a pattern from apatterning device onto a substrate using an electro-magnetic radiationbeam, comprising: a data storage device arranged to receive and storedata characterizing a birefringence property of the patterning device, amanipulator for adjusting one or more birefringent elements in usetraversed by said radiation beam, a controller responsive to said dataand arranged to control said adjusting to compensate impact of thepatterning device on a polarization state of said radiation beam atsubstrate level.
 2. A lithographic apparatus according to claim 1whereby said birefringence property of the patterning device is a phaseretardation.
 3. A lithographic apparatus according to claim 2 wherebysaid adjusting comprises varying a phase retardation or selecting aphase retardation of said one or more birefringent elements.
 4. Alithographic apparatus according to claim 1 whereby said birefringenceproperty of the patterning device is a direction of a fast axis.
 5. Alithographic apparatus according to claim 4 whereby said adjustingcomprises changing a direction of a fast axis of said one or morebirefringent elements.
 6. A lithographic apparatus according to claim 1whereby the apparatus is a step-and-scan apparatus, and whereby themanipulator is arranged to adjust a plurality of elongated birefringentelements, said birefringent elements having a common direction ofelongation, and being disposed parallel to said common direction ofelongation, and being mutually displaced with respect to each other in adirection perpendicular to said common direction of elongation, eachbirefringent element being movable along its direction of elongation,and each birefringent element having a non-uniform distribution of oneor more birefringence properties along the axis of elongation.
 7. Alithographic apparatus according to claim 6 whereby said non-uniformdistribution is a distribution of phase retardance.
 8. A lithographicapparatus according to claim 6 whereby said non-uniform distribution isa distribution of directions of a slow axis.
 9. A lithographic apparatusaccording to claim 1 further comprising a birefringence measuring devicefor measuring birefrigence of the patterning device and for providingsaid data characterizing a birefringence property of the patterningdevice.
 10. A device manufacturing method comprising projecting apattern from a patterning device onto a substrate using anelectro-magnetic radiation beam, the method comprising: storing datacharacterizing a birefringence property of the patterning device,adjusting one or more birefringent elements in use traversed by saidradiation beam, controlling said adjusting to compensate impact of thepatterning device on a polarization state of said radiation beam atsubstrate level.